Going through I like this the best, but a couple of criticisms. Too much foreground, need to reframe or crop that out, and I would like to have seen the top of the dustcloud. Slower shutter speeds and panning gives a better impression of speed. This takes practice so get a friend to drive a car and pan with the car, pressing the release while still panning. You can use speeds as low as 1/30th when you get good at it, but speeds around 1/60th or 125th may be a good starting point. Also consider a longer lens for your Pentax, lets you get back a bit...safer and your pride and joy won't end up covered in grit!